The invention relates to a mixture which comprises    a) from 95 to 30% by weight of a copolymer or block copolymer A comprising one or more copolymer blocks (B/S)A which are respectively composed of from 65 to 95% by weight of vinylaromatic monomers and of from 35 to 5% by weight of dienes, and whose glass transition temperature TgA is in the range from 40° to 90° C.,    b) from 5 to 45% by weight of a block copolymer B comprising at least one hard block S which is composed of vinylaromatic monomers, and comprising one or more copolymer blocks (B/S)B which are respectively composed of from 20 to 60% by weight of vinylaromatic monomers and of from 80 to 40% by weight of dienes, and whose glass transition temperature TgB is in the range from −70° to 0° C.,    c) from 0 to 40% by weight of polystyrene C, and    d) from 0 to 30% by weight of a block copolymer D comprising at least one copolymer block (B/S)A which is respectively composed of from 65 to 95% by weight of vinylaromatic monomers and of from 35 to 5% by weight of dienes, and whose glass transition temperature TgA is in the range from 40° to 90° C., and comprising at least one diene block B or one copolymer block (B/S)B which is respectively composed of from 1 to 60% by weight of vinylaromatic monomers and of from 99 to 40% by weight of dienes, and whose glass transition temperature TgB is in the range from −100° to 0° C.,    e) from 0 to 6% by weight of a plastifying agent, and its use for the production of packaging for electronics components.
U.S. Pat. No. 3,639,517 describes star-shaped branched styrene-butadiene block copolymers having from 75 to 95 percent by weight of terminal blocks composed of vinylaromatic monomers and from 5 to 30 percent by weight of elastomeric blocks mainly composed of conjugated diene units. They can be blended with standard polystyrene to give highly transparent mixtures. As the proportion of polystyrene increases, the modulus of elasticity increases at the cost of toughness. Mixtures with as little as about 40 percent by weight of polystyrene are too brittle for most applications. The possible amount of polystyrene admixed is mostly only from 20 to a maximum of 30 percent by weight, if acceptable ductility is to be retained.
WO 00/58380 describes star-shaped block copolymers having 40% by weight of hard blocks composed of vinylaromatic monomers and soft blocks having random structure composed of vinylaromatic monomers and dienes. To increase stiffness, they are blended with standard polystyrene, whereupon transparency falls. Even with 60 percent by weight of polystyrene they still give ductile mixtures. The disadvantage of these blends is clearly visible haze which is unacceptable for more demanding applications and components of relatively high thickness.
WO 2006/074819 describes mixtures of from 5 to 50% by weight of a block copolymer A comprising one or more copolymer blocks (B/S)A which are respectively composed of from 65 to 95% by weight of vinylaromatic monomers and of from 35 to 5% by weight of dienes, and whose glass transition temperature TgA is in the range from 40° to 90° C., and from 95 to 50% by weight of a block copolymer B comprising at least one hard block S which is composed of vinylaromatic monomers, and comprising one or more copolymer blocks (B/S)B which are respectively composed of from 20 to 60% by weight of vinylaromatic monomers and of from 80 to 40% by weight of dienes, and whose glass transition temperature TgB is in the range from −70° to 0° C., for the production of shrink films. The stiffness of the mixtures is in the range from 700 to at most 1300 MPa.
Blending of conventional styrene-butadiene block copolymers, such as Styrolux®, with polystyrene can be used to adjust the modulus of elasticity as desired up to above 3000 MPa, as a function of mixing ratio. However, experience has shown that there is a drastic loss of ductility at a modulus of elasticity above 1900 MPa. The mechanical behavior of the mixtures is then similar to that of polystyrene itself, and the mixtures lose their advantages over polystyrene.
Packaging materials for electronic components, for example tubes in which integrated circuits are transported, require a combination of high stiffness and ductility in association with safe exceeding of the yield stress, and good transparency. Polystyrene and its mixtures with styrene-butadiene block copolymers have not hitherto been suitable for these applications. The market has hitherto been serviced by polyvinyl chloride (PVC) or very expensive specialty polymers.